The precise measurement of thicknesses, mechanical properties, and film-substrate bonding qualities of sub-micron metallic films, multi-layers, and microstructures in a nondestructive way is of growing importance for the semiconductor industry as well as for MEMS manufacturers.
Short laser pulses having durations of less than 100 fs are an ideal – not to say the only –way in order to provide sub-surface visualization in nanometer scale perpendicular to the film surface.
In this investigation, an enhanced pump-probe set up is presented. A dual frequency modulation technique is implemented in order to eliminate optical and electrical cross talk between the excitational pump pulse and the measuring probe pulse.
Up to the 5th acoustic echo in an aluminum film having a thickness of 50 nm on a sapphire substrate has been measured. From the time of flight and from the amplitude decay between the echoes the material properties of the film and of the substrate have been derived.
Further directions of the on going research project are presented.